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Organic N (oN, e.g., amino acids) is an important N-resource for plants in soils replete with oN but not inorganic N (iN; i.e., NH4+ and NO3−), such as cold ecosystems with temperature-limited soil decomposition rates. However, sub-Antarctic literature assumes that plants only acquire iN, potentially underestimating plant-available N. We hypothesised that Marion Island (− 46.90°, 37.75°) grasses (Polypogon magellanicus, Poa cookii, Agrostis stolonifera and Poa annua) acquire oN and that oN relative to iN provision affects plant growth. We investigated oN and iN uptake and growth responses in two hydroponics experiments. In situ N (15N-glycine, 15NO3− and 15NH4+) acquisition was investigated at three field sites with decreasing faunal influence, thus iN input and microbial activity. When plants grown in mire water were supplied with 15N-glycine or 15NO3−, root δ15N enrichment was highest for glycine-supplied plants. In the second hydroponics experiment, plant N-uptake rates (nmol g biomass−1 s−1) were significantly higher for glycine than NO3−, but relative growth rates (g g−1 d−1) lower on glycine. There were species-specific biomass allocation responses to N concentration (4 mM and 0.4 mM) and N-form (glycine and NO3−). Glycine-supplied grasses at the low iN concentration field sites had significantly higher δ15N enrichment relative to those at sites with high iN, suggesting higher oN uptake when iN is limiting. We demonstrate the importance of accounting for oN acquisition in the sub-Antarctic. As a system with high soil oN relative to iN, plants may predominantly meet N-demands through oN rather than iN acquisition. [ABSTRACT FROM AUTHOR]

Islands of fertility associated with tree/shrub patches in arid grasslands create spatial heterogeneity of soil nutrients. Faunal activities under these trees/shrubs may contribute to diverse characteristics of these fertile patches of soil due to different faecal inputs. This study used the tree islands of fertility (TIFs) created by camelthorn (Vachellia erioloba) and shepherd trees (Boscia albitrunca) and these species that also host sociable weavers (Philetairus socius) nests (bird islands of fertility; BIFs) in the Kalahari Desert to investigate the diversity in islands of fertility. We hypothesized that grasslands, the TIFs, and the BIFs differ in both the concentrations and stoichiometries of soil nutrients due to the sources of nutrients and that this subsequently determines the growth and foliar nutrient concentrations and stoichiometries of plants that grow on these soils. We predicted that seedlings grown on soils from BIFs would have greater growth, higher biomass yield, and higher foliar nutrients than those grown on soils from TIFs and grasslands. We sampled and grew camelthorn seedlings in soils from BIFs, TIFs, and matrix grasslands. Despite the higher soil nutrients in TIFs than in grasslands, there were no significant differences in seedling growth. However, we observed significantly higher seedling growth in BIF soils compared to TIF soils. Seedlings grown in soils from BIFs and grasslands allocated more growth to shoots and roots, respectively, while those grown on TIF soils were intermediate. The foliar nutrient stoichiometries of seedlings grown in BIF soils were similar to the stoichiometric ratios in BIF soils and in sociable weaver faecal matter. This shows that the faecal input of the sociable weaver accounts for the growth differences in these islands of fertility. The ecological engineering activities of the sociable weaver address nutrient limitations for camelthorn seedling growth, which TIF soils could not address despite the high faecal input of mammals. [ABSTRACT FROM AUTHOR]

Regulation of alien invasive species by indigenous generalist herbivores is a strong component of ecosystem biotic resistance. Globally, Australian Acacia species are highly invasive trees that are regulated by specialist invertebrate herbivores, granivorous birds and consumption by indigenous and exotic vertebrates in their native ranges. Australian Acacias have become highly invasive in the Cape Floristic Region (CFR) of South Africa where seeds are consumed by generalist bird, ant, and rodent species, but there is as yet no evidence that generalist herbivores consume the seedlings. Despite this, indigenous mice are common in the CFR and we consequently hypothesised that seedling herbivory by indigenous herbivores may limit survival of Acacia cyclops in relatively undisturbed indigenous ecosystems. Acacia cyclops seedlings were transplanted into indigenous CFR coastal "strandveld" vegetation at two distinct study sites and monitored for eight days in one of three caged treatments (i) no cage, (ii) partially caged excluding large mammalian herbivores or (iii) fully caged excluding all mammalian herbivores. Herbivory-induced seedling mortality without a cage and for partially caged plants was significantly greater than for fully caged plants (all contrasts P < 0.001). Rapid herbivory of A. cyclops seedlings in the uncaged and partially caged treatments was largely attributable to highly abundant striped mice, Rhabodmys pumilio. Seedling herbivory was not dependent on rodent density, which was consistently high (200–300 mice ha−1). However, seedling herbivory was higher in areas with dense vegetation and lower in open-canopy areas, probably because R. pumilio is known to avoid foraging in open areas. These generalist herbivores reduced A. cyclops recruitment in relatively undisturbed indigenous ecosystems, contributing to a degree of ecosystem resistance. [ABSTRACT FROM AUTHOR]

Female plants not only flower but also produce resource-rich seeds, fruits, and cones. Thus, it is generally considered that female plants allocate more resources to sexual reproduction than male plants and that this allocation difference can explain vegetative dimorphism, such as greater leaf size in females. We found significant sexual vegetative differences in the dioecious and serotinous species, Aulax umbellata and A. cancellata. Plant height, annual branch length and canopy spread were greater in males whereas leaf size, branch thickness and branch number were greater in females. Sex ratios and basal stem area were, however, equal in the sexes. Equal sex ratios imply equal allocation to sexual reproduction and equal stem areas imply equal resource use and biomass, and thus allocation to vegetative growth. Given equal allocation to reproduction and resource use, we suggest that the vegetative dimorphism is driven by intra-male-competition to be more visually conspicuous to pollinators. This implies that plant architecture is both a vegetative and a reproductive trait. [ABSTRACT FROM AUTHOR]

Background and aims: Plant species richness increases with declining soil phosphorus (P) availability, especially for Proteaceae in old infertile landscapes. This difference in richness might be attributed to faster diversification in lineages adapted to P-impoverished soils, i.e. species that possess specialised P-acquisition strategies, and have lower leaf P concentration ([P]) and higher seed [P]. Alternatively, a longer time for species accumulation might contribute to high species richness in low-P environments due to the geological stability of the landscapes in which they evolved. Methods: We assessed differences in diversification of Proteaceae in P-impoverished vs. nutrient-rich environments and whether these were linked to adaptations to P-impoverished soils. We explored mature leaf and seed [P] and investigated how these traits changed over the evolutionary history of the family, and within two species-rich genera (Banksia, Hakea). Results: Faster diversification was correlated with lower leaf and higher seed [P] for species-rich genera across the Proteaceae. For Banksia and Hakea, diversification rates peaked at relatively low leaf [P], but not at the lowest leaf [P]. Ancestral state reconstructions indicated that low leaf [P] is a trait that was likely present in the early evolution of the Proteaceae, with recent transitions to higher leaf [P] across several species-poor rainforest genera. Conclusions: Diversification of Proteaceae correlated strongly with P-related traits. In an evolutionary context, functional cluster roots, low leaf [P] and high seed [P] were likely key innovations allowing diversification. Selection for low leaf [P] early in the evolutionary history of Proteaceae pre-adapted ancestors of this family to diversify into oligotrophic environments. We discuss how our findings are likely relevant for understanding diversification dynamics of other plant families that occur in P-impoverished environments. [ABSTRACT FROM AUTHOR]

Fairy circles are striking regularly sized and spaced, bare circles surrounded by Stipagrostis grasses that occur over thousands of square kilometres in Namibia. The mechanisms explaining their origin, shape, persistence and regularity remain controversial. One hypothesis for the formation of vegetation rings is based on the centrifugal expansion of a single individual grass plant, via clonal growth and die-back in the centre. Clonality could explain FC origin, shape and long-term persistence as well as their regularity, if one clone competes with adjacent clones. Here, we show that for virtually all tested fairy circles the periphery is not exclusively made up of genetically identical grasses, but these peripheral grasses belong to more than one unrelated genet. These results do not support a clonal explanation for fairy circles. Lack of clonality implies that a biological reason for their origin, shape and regularity must emerge from competition between near neighbor individuals within each fairy circle. Such lack of clonality also suggests a mismatch between longevity of fairy circles versus their constituent plants. Furthermore, our findings of lack of clonality have implications for some models of spatial patterning of fairy circles that are based on self-organization. Christian Kappel et al. examine the genetic composition of fairy circles, regular circular patterns of grasses in the Namib Desert, using ddRAD-seq. They find that these grasses are made up of multiple unrelated genets rather than genetically identical grasses, suggesting non-clonality. [ABSTRACT FROM AUTHOR]

Background and aims: Although a number of different factors influence C and N isotopic fractionation of organic matter, the δ13C and δ15N values of soil organic matter both tend to increase with soil depth, following similar trajectories. This similarity has not been investigated at the global scale. As microbial decomposition increases organic matter δ13C and δ15N values, soil isotopic values are hypothesized to generally increase with depth across local and global scales.Methods: Soil δ13C and δ15N values for 16 soil depth-profile sites were used for local-scale investigation, and 5447 global single-depth sites were used for global-scale investigation of the correspondence between δ13C and δ15N. Correlative and boosted regression tree analyses were used to determine the main drivers of the variance in soil δ15N globally and also the environmental association of variability in the correlation with depth between δ13C and δ15N at a number of sites.Results: Strong positive correlations between δ13C and δ15N values through soil profiles were found at a number of sites and were found to be independent of vegetation type. Globally, soil δ13C and δ15N values were also found to be significantly positively correlated across a wide range of climates and biomes.Conclusion: The global correspondences between δ13C and δ15N values may suggest a mechanistic link between δ13C and δ15N through the process of SOM decomposition and microbial processing and highlight the importance of soil-related processes in determining isotopic signals in soils. The variability in these soil processes should be considered when interpreting soil isotopic values of δ13C and δ15N as indicators of ecosystem sources of soil C and N and inferring vegetation inputs. [ABSTRACT FROM AUTHOR]

Background and aims: Legume species in the fynbos vegetation of the Cape Floristic Region, that fix N 2 in soils with low P, may have evolved for enhanced acquisition and efficient use of P. It was hypothesized that N 2-fixing and combined-N supplied (N-supplied) A. linearis, P. calyptrata and C. genistoides are adapted to low P and would be relatively unresponsive to increased P of 100 μM. Methods: 18 legume species were evaluated for their nodulation response to low P availability. The N X P interaction was then examined in A. linearis, P. calyptrata and C. genistoides reliant on either N 2-fixation or 300 μM N (NH 4NO 3), and receiving 0.1, 1.0, 10 and 100 μM P (NaH 2PO 4). Results: In the species selection experiment, A. linearis, P. calyptrata and C. genistoides, with the greatest nodule fresh weight (FW) and nodule FW to root FW ratio, were the most prolific nodulating species. In the N X P experiment, with low P supply, the biomass of N 2-fixing P. calyptrata and C. genistoides was consistently greater than that of N-supplied plants. In contrast, with high P supply of 100 μM P, all N-supplied plants accumulated more biomass than the corresponding N 2-fixing plants. High P-use efficiency, poor down-regulation of P uptake and P storage was evident in A. linearis and P. calyptrata. Conclusion: The growth response to P and the significant N X P interactions indicate that N 2-fixing and N-supplied plants were not adapted to low P, but rather colimited by both N and P. [ABSTRACT FROM AUTHOR]

Tree-on-tree competitive interactions may be more important in affecting the distribution of the tree components of savannas than inter-specific competition with grasses. The presence of intraspecific competition is expected to negatively affect inter-tree spacing, individual size distributions and plant physiology as well as survival/mortality. In this field removal experiment on Acacia mellifera, one of South Africa's most common encroachers on nutrient-poor soils, the growth, water relations and mortalities of shrubs where all neighbouring woody competitors were removed (target) were monitored three times during each of three growing seasons. After 3 years, the nitrogen and carbon isotopic ratios of the study plants were analysed. Target shrubs benefitted from removal of neighbours, resulting in greater growth, less water stress, a relatively small degree of canopy dieback and reduced reliance on N fixation. Target shrubs grew by 25 ± 4% in height relative to 7 ± 4% for controls, with the targets suffering a maximum of <15% canopy dieback compared to up to 60% in the controls. Severe environmental stress is known to affect neighbour interactions among shrubs and competition may constrain shrub sizes and avoid density-dependent mortality. In contrast, release from competition in our study may have allowed greater growth of target plants, increasing their total evapo-transpirational leaf surface areas and leaving them vulnerable to drought and water stress. Intratree competition on shallow nutrient-poor soils in savannas may thus aid the persistence of bush encroachment by regulating the sizes of individual shrubs below the threshold of drought vulnerability. [ABSTRACT FROM AUTHOR]

with many grasslands globally, the Highveld grasslands of South Africa are tree-less, despite having a climate that can support tree growth. Models predict that fire maintains these grasslands. The question arises as to why fire-tolerant savanna trees do not survive in these ecosystems? Savanna tree survival in mesic areas is restricted by demographic bottlenecks, specifically limitations to sapling-escape from fire. It was hypothesised that ancient highly leached soils from grassland areas would prevent saplings from growing fast enough to escape the fire-trap. Growth rates of savanna tree seedlings ( Acacia karroo Hayne and Acacia sieberiana Burtt Davy) were measured in a common garden experiment using soils from ten sites collected along a savanna-grassland continuum. Soils from grassland sites were relatively nutrient-poor compared to those from savannas with lower pH, and associated cations. A. sieberiana growth rates responded to pH and these nutrients, whereas A. karroo growth was less strongly linked to specific nutrients. Even so, both species accumulated more biomass when grown in soils from savanna sites compared to grassland sites. An exception was a low elevation low nutrient savanna site that resulted in poor growth, yet sustains high tree biomass in situ. Differences between growth in grassland and savanna soils were small. They may contribute to, but are unlikely to explain, the treeless nature of these grasslands. [ABSTRACT FROM AUTHOR]

undance of Fabaceae declines in representation through post-fire-succession in fynbos vegetation of the Cape Floristic Region (CFR). This reduction in legume occurrence coincides with a known decline in post-fire soil P availability. It was hypothesized that the disappearance of legume species during post-fire succession is due to an inability to acquire P effectively from sparingly soluble sources. P-acquisition strategies and response to P supply were compared between legume ( Aspalathus, Cyclopia, Indigofera, Podalyria) and non-legume ( Elegia, Leucadendron, Protea) genera when supplied with 1 or 10 mg P kg dry sand. Each genus consisted of a seeder (non-persistent) and resprouter (persistent) species. Non-legumes showed a greater investment in below-ground biomass, more root clusters, with higher concentrations of carboxylates exuded by cluster roots and carboxylates that were better suited to the mobilization of sparingly soluble P compared to legumes. The growth response to increased P supply was 53% higher in legumes than in non-legumes. The lack of a growth response to an elevated P supply in the non-legumes was attributed to N-limitation. Legume resprouters had a higher investment in cluster-root biomass and a lower capacity to down-regulate P-uptake than the seeders. Therefore the inability to acquire sufficient P from low concentration and sparingly soluble soil P-sources may contribute to the lack of indigenous legume persistence in fynbos vegetation of the CFR. [ABSTRACT FROM AUTHOR]

The article discusses various reports published within the issue, including one on data showing that foliar [N], [P] and N:P ratios of fynbos and west-Australian kwongan plants are relatively similar, another on the plants native to kwongan have seeds with 2.2-fold higher and 4.7-fold greater phosphate contents than those of fynbos plants and another on the importance of phosphate in determining the biodiversity and structure of vegetation.

Transpiration is generally considered a wasteful but unavoidable consequence of photosynthesis, occurring because water is lost when stomata open for CO2 uptake. Additionally, transpiration has been ascribed the functions of cooling leaves, driving root to shoot xylem transport and mass flow of nutrients through the soil to the rhizosphere. As a consequence of the link between nutrient mass flow and transpiration, nutrient availability, particularly that of NO3 −, partially regulates plant water flux. Nutrient regulation of transpiration may function through the concerted regulation of: (1) root hydraulic conductance through control of aquaporins by NO3 −, (2) shoot stomatal conductance ( gs) through NO production, and (3) pH and phytohormone regulation of gs. These mechanisms result in biphasic responses of water flux to NO3 − availability. The consequent trade-off between water and nutrient flux has important implications for understanding plant distributions, for production of water use-efficient crops and for understanding the consequences of global-change-linked CO2 suppression of transpiration for plant nutrient acquisition. [ABSTRACT FROM AUTHOR]

In frequently burnt mesic savannas, trees can get trapped into a cycle of surviving fire-induced stem death (i.e. topkill) by resprouting, only to be topkilled again a year or two later. The ability of savanna saplings to resprout repeatedly after fire is a key component of recent models of tree–grass coexistence in savannas. This study investigated the carbon allocation and biomass partitioning patterns that enable a dominant savanna tree, Acacia karroo, to survive frequent and repeated topkill. Root starch depletion and replenishment, foliage recovery and photosynthesis of burnt and unburnt plants were compared over the first year after a burn. The concentration of starch in the roots of the burnt plants (0.08 ± 0.01 g g−1) was half that of the unburnt plant (0.16 ± 0.01 g g−1) at the end of the first growing season after topkill. However, root starch reserves of the burnt plants were replenished over the dry season and matched that of unburnt plants within 1 year after topkill. The leaf area of resprouting plants recovered to match that of unburnt plants within 4–5 months after topkill. Shoot growth of resprouting plants was restricted to the first few months of the wet season, whereas photosynthetic rates remained high into the dry season, allowing replenishment of root starch reserves. 14C labeling showed that reserves were initially utilized for shoot growth after topkill. The rapid foliage recovery and the replenishment of reserves within a single year after topkill implies that A. karroo is well adapted to survive recurrent topkill and is poised to take advantage of unusually long fire-free intervals to grow into adults. This paper provides some of the first empirical evidence to explain how savanna trees in frequently burnt savannas are able to withstand frequent burning as juveniles and survive to become adults. [ABSTRACT FROM AUTHOR]

Presents an introductory statement to the January 2003 issue of the journal, "Plant and Soil."

Quantifying global patterns of terrestrial nitrogen (N) cycling is central to predicting future patterns of primary productivity, carbon sequestration, nutrient fluxes to aquatic systems, and climate forcing. With limited direct measures of soil N cycling at the global scale, syntheses of the 15N:14N ratio of soil organic matter across climate gradients provide key insights into understanding global patterns of N cycling. In synthesizing data from over 6000 soil samples, we show strong global relationships among soil N isotopes, mean annual temperature (MAT), mean annual precipitation (MAP), and the concentrations of organic carbon and clay in soil. In both hot ecosystems and dry ecosystems, soil organic matter was more enriched in 15N than in corresponding cold ecosystems or wet ecosystems. Below a MAT of 9.8°C, soil δ15N was invariant with MAT. At the global scale, soil organic C concentrations also declined with increasing MAT and decreasing MAP. After standardizing for variation among mineral soils in soil C and clay concentrations, soil δ15N showed no consistent trends across global climate and latitudinal gradients. Our analyses could place new constraints on interpretations of patterns of ecosystem N cycling and global budgets of gaseous N loss. [ABSTRACT FROM AUTHOR]